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The root-fungi of orchids 

Burgeff 1 has brought together the extended results of his own research 
and those of other students upon the root-fungi of orchids. The volume con- 
tains a comprehensive citation of literature, several tables summarizing results 
of experiments, and discussions of the theory of the mycorhiza question. By 
way of introduction, Burgeff defines his use of the word symbiosis as that 
relation of two symbionts in which one aids the other in any way, if only to 
make existence possible under conditions otherwise impossible. After union, 
the two organisms form a new organism, a unit, which takes up the struggle 
for existence under new conditions, each being a body member. Such is claimed 
to be the condition of the orchids and their root-fungi. The two sections of 
the work are as follows: 

i. The study of the fungus independent of the plant, — On a culture medium 
of agar and rain water with a slight trace of starch, 29 root-fungi, aseptically 
obtained from native and tropical orchids, were grown (holosaprophytic ones 
were unsuccessful). Such species of Orcheomyces, as the author chooses to call 
the fungi for convenience, are described in detail as to their structure and 
behavior in the culture. For the first time a study of their enzymes has been 
made, and the endophytes were grouped accordingly on the basis of their 
biological relations to the orchids. In general they have thin-walled, regularly 
septate mycelia; the hyphae are sharply differentiated into Langhyphen that 
branch little and show unlimited growth in one direction, and Kurzhyphen that 
are of smaller caliber and arise at regular intervals and whose cells under 
certain conditions are transformed into spores (Sporentrager) or absorptive 
hyphae (Sanghyphen). In old age spores and hyphae contain a fatty oil. 
Chains of hyaline or slightly colored spores may unite into loose clusters or 
closer sclerotia-like groups. All fungi fuse or anastomose in some manner. 
The hyphal cells contain two to ten nuclei, but the spores contain only two 
nuclei. Spiral knots initiated in cultures by the surface tension of a drop of 
water at the tip of the hypha are comparable to those in the root cells developed 
in response to the pressure of the resisting plasma membrane to the penetration 
of the hypha. No sexual reproduction was observed. 

1 Burgeff, Hans, Die Wurzelpilze der Orchideen; ihre Kultur und ihr Leben in 
der Pflanze. 8vo. pp. 207. pis. 3. figs. 38. Jena: Gustav Fischer. 1909. 

, Zur Biologie der Orchideen Mycorhiza. pp. 66. Inaug. Diss. Jena: 

Gustav Fischer. 1909. 



Of the carbohydrate culture media, starch, maltose, and saccharose were 
best; glucose and dextrin next; and glycerin poorest. Correspondingly, all 
fungi possessed diastase and emulsin; some invertase and maltase; some 
aesculin; one tyrosinase; and only one cytase. In humus decoctions all 
grew well. 

The endophytes cannot assimilate free nitrogen. They belong to Ben- 
necke's category of "Ammon-nitrit-nitratpilze," and grow luxuriantly, form- 
ing spores on ammonium salts, ammonium nitrate holding first place. Organic 
N compounds were varyingly well assimilated. Peptone produced splendid 
growth, and salep furnished sufficient N for all to grow. All fungi possess 
proteolytic enzymes. 

The production of acid in assimilation is slight, and in general, in media 
containing asparagin, peptone, or urea, is in proportion to the intensity of the 
growth. Orchid fungi require to a high degree atmospheric oxygen, dying 
in long-continued anaerobic cultures. The formation of spores and spore 
sclerotia depends upon the concentration of the medium, its exhaustion, and 
the amount of assimilation products present. Salep always stimulated spore- 
formation, as did increased transpiration. 

2. The study of the plant and the fungus. — For the biological relations of 
the plant and the fungus, Burgeff chooses the terrestrial orchid Epidendrum 
(dichromum?) and the hybrid epiphyte Laelio-Catlleya. Without a fungus, 
seeds of Epidendrum, on a culture medium of rain water, 2 per cent salep, and 
1 . 5 per cent agar, did not germinate. With ten different fungi they germinated 
within 25 days. The embryo did not become green until infected. Thinned 
nutritive agar cultures with fungi produced stouter plants than the rain water 
cultures. The stages of development and limits of the fungus agree with the 
Cypripedium type of development described by Bernard. Infection takes 
place through the suspensor at first, and later through the root hairs, passing 
by means of the Durchlasszellen into the cortex. The hyphae of the sub- 
epidermal layer (Pilzwirtzellschicht) are never digested, but Eiweisshyphen 
appear in the digestive layer beneath. The cytological facts agree with the 
phenomena of digestion, where the remains (a clump) are surrounded by a 
cellulose layer (Haut), and the nucleus assumes the resting stage, ready to 
digest new hyphae. Clumps may become several-layered. 

Burgeff found that the seeds of Laelio-Cattleya, wettable only after three 
or four days, could germinate, become green, and attain considerable differ- 
entiation (stomata at apex, rhizoids at base, leaf primordia) in three or four 
months. A resting period of a year is necessary before growth continues, for 
which also a fungus is required. Just before infection, the oil in reserve in 
the cells near the suspensor is transformed to starch, which disappears, however, 
as soon as the fungus penetrates the cells. Both embryo and fungus show 
evidence of growth, the latter by the formation of Eiweisshyphen (homolo- 
gous with spores and hyphal knots) . 

Testing the possibility of saprophytic germination in Laelio-Cattleya, 


Burgeff obtained its development, in 0.33 per cent cane sugar with mineral 
salts on agar, in the dark, to the differentiation of papillae, and it lived ten 
months. Further development either in light or dark required the fungus. 
While in the light the orchid deve loped normally, in the dark it elongated and 
resembled the Epidendrum which grew in the weaker concentration of salts. 
Burgeff concludes that physio logi cally the behavior of the fungus is alike 
in the germination of the epiphyte and the terrestrial orchid, and that the 
appearance of chlorophyll in the epiphyte is an adaptation to its life in the 

Seedlings with the fungus in absence of C0 2 in the dark grew, but developed 
no root, while in the light the growth was normal. Experiments on the physi- 
ology of nutrition, where fungus and seedling were grown together, showed the 
best N sources to be ammonium chlorid and nitrate. Although asparagin was 
favorable for the fungus in free culture, the plant could not grow in it. In 
N-free media there was no development. Lactic acid (2 per cent) in mineral 
salt agar culture caused both plant and fungus to grow well. Laelio-Cattleya 
was found to harbor some fungi (17 tested) unable to stimulate its germination- 

Burgeff devotes 50 pages to the histological processes in the growing 
plants. These detail the places of entrance, position of the fungi, peculiar 
features in the plant cell (spores and clumps, Klumpenbildung) , the emission 
of hyphae to the substratum, etc. Most of the fungi have mycelial connections 
with the outside substratum, therefore anatomically there is nothing to prevent 
ascribing to such hyphae the function of conduction of soluble materials. The 
author explains the unwettability of the seeds as due to the Luftblaschen in the 
netted testa itself, and to the air between the testa and embryo (comparable 
to the condition in the lycopod spores). Such unwettability hinders the pas- 
sage of spores through the soil, contrary to the theory of Koch and LOstner, 
but is of advantage in preventing the clinging together of seeds in the capsule, 
therefore an adaptation to their dissemination by wind. Also because the seeds 
are chemically attracted to the fungi, the unwettability is an adaptation in 
terrestrial orchids against too rapid exhaustion of this substance. 

Burgeff agrees with Bernard in the probable steps of development of 
the symbiosis by way of parasitism, but would speak of the association not as 
a "maladie bienfaisante," but as of "einem gliicklichen Zusammentreffen 
verschiedener Umstande." He bases this remark on the following facts: the 
fungus is harmless, its enzymatic qualities separating it from comparison with 
parasites with toxic qualities; it is able to kill an unadapted fungus whose 
penetration it nevertheless allows; its fitness for infection is seen in the 
Durchlasszellen of the embryo. A mutualistic symbiosis demanded by the 
definition is found in this association, for the orchid has to thank the fungus for 
its existence, and the fungus is grateful to the orchid for the materials difficult 
to obtain from the soil which enable it to form spores. As to the materials 
of exchange between the components, the question is left unanswered; but the 
idea of conduction of mineral salts is favored, because of the results of the 


cultural experiments and because of the detailed morphological and anatomical 
features of both symbionts. The fungus causes the conversion of starch into 
sugar by its diastase. Its function results from its enzymatic quality, which, 
with the solution of carbohydrates in the plant cell, induces the development 
of the seed, not by bringing soluble materials to the cell, but by transforming 
substances already there. Burgeff suggests here the unproven fact of diffu- 
sion of the diastatic enzyme out of the fungal hypha through the Plasmahaut 
into the plant cell. This may also occur in the substratum from the emission 
hyphae. The osmotic relations arising from the sugar solutions could account 
for the absorption of water, but if nutritive salts are absorbed from the fungi 
from outside, a rapid change in permeability and adjustment of pressures at 
just the proper time to seize the salts brought by the fungus must take place. 
On the whole, the relations between the plant and the mineral salts of the 
soil are of striking importance for the origin and maintenance of the orchid 
symbiosis. Although the structures show a gain in nitrogenous substances, 
the habitats of orchids, and cultural experiments exclude the possibility of free 
N-absorption. No anatomical features can prove the absorption of organic 
carbohydrates; although diastase and emulsin are common to all fungi, material 
for the action of the former is lacking in the soil, and we are in ignorance con- 
cerning the substance in the soil digested by the latter. Any substance taken 
up by the plant, either through its roots or by means of the fungus, must first 
be made soluble by the fungus itself, or by its exoenzymes in the substratum. — 
Grace L. Clapp. 


Farm weeds. — The preparation of a scientific manual for the use of the 
ordinary layman is admittedly a difficult task, but it has been successfully 
accomplished by Clark and Fletcher, 2 whose volume upon farm weeds is the 
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the more troublesome weeds of Canada are arranged according to modern 
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and briefly but accurately described in non-technical language. Special 
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troublesome as weeds, and careful directions are given for the most prac- 
ticable and successful methods of control and extermination. 

The most valuable aid to the recognition of different species is a series 
of 76 full-page plates, colored with the greatest accuracy. They include 

2 Clark, George H., and Fletcher, James, Farm weeds of Canada. Second 
edition. 8vo. pp. 192. ph. 76. Ottawa: Department of Agriculture, Dominion of 
Canada. 1909. $1.00 (single copies only, for sale by Superintendent of Stationery, 
Government Printing Bureau, Ottawa).